The present invention relates to a knock detecting apparatus adapted for use with internal combustion engine ignition timing control systems, etc. More particularly, the present invention is directed to such systems which are designed to control engine timing so that knock, detected from the vibrations produced outside the engine cylinders due to the cylinder pressure, is controlled to a desired intensity.
As is well known in the art, there is a close correlation between ignition timing and cylinder pressure. When an air-fuel mixture is exploded, in the absence of knocking there is no superposition on the cylinder pressure of any higher harmonic component. This higher component usually has a frequency in the range of 5 to 10 kHz which falls in the frequency band determined by the cylinder bore diameter and the velocity of sound in the combustion and which is caused by the intermittent and rapid combustion) If knocking starts to occur such higher harmonic component starts to superpose on the cylinder pressure at around the maximum cylinder pressure value and this results in the generation of vibrations or sound outside the cylinders. Careful examination of the pressure signals generated inside the cylinders and the generation of vibrations or sound outside the cylinders shows that the beginning of knocking (trace knock) starts to occur at an engine crank angle at which the cylinder pressure attains the maximum value and that as the knock intensity gradually increases (to light knock and heavy knock) the higher harmonic component starts to superpose considerably earlier (or on the ignition side) than the maximum cylinder pressure crank angle. While the engine efficiency will be increased greatly if the vibrations or sound emitted to the outside of the cylinders by the knocking is detected accurately and fed back so as to control the ignition timing, there has been in fact no detecting apparatus capable of operating stably under severe environmental conditions in which vehicles are run.
A known detecting apparatus of the above type employs a piezoelectric element type acceleration detector for the purpose of detecting the engine vibrations due to knocking. However, this type of knock detecting apparatus has many deficiencies with respect to the following essential requirements for engine knock detecting apparatus.
(1) Since the engine knock increases as the ignition timing is advanced as mentioned previously, the desired knock control level for ignition timing controlling purposes must be lower than the trace knock level and a highly accurate and stable detecting characteristic is an essential requisite. However, the piezoelectric element is such that its impedance is essentially high, its characteristics tend to be affected by humidity and contamination and it is difficult to generate stably a small signal indicative of the trace knock (namely, the element is large in drift and amplitude variation). Further, while the piezoelectric properties of the element are provided by polarization, due to the fact that the element is used under severe conditions which cyclically change through high temperature to low temperature conditions, this polarization tends to be lost gradually with the resulting decrease in the sensitivity. The decreased sensitivity causes a rise in the knock control level which in turn causes damages to the engine due to the knock.
(2) It is desirable that the detecting apparatus is inexpensive to manufacture. However, since the piezoelectric element has a high impedance, its output signal must be amplified by means of an expensive amplifier of the high impedance input type which is called as a charge amplifier. Also, the high impedance type amplifier tends to malfunction under the effects for example of noise due to the ignition signals of an engine and the prevention of such malfunction requires the provision of a more expensive and complicated construction.
(3) The use of the piezoelectric element as a vehicle-mounted sensor involves many problems from the standpoint of durability, cost, etc., in that it is low in shock resistance, tends to produce cracks, breaking or the like and requires the use of cooling means when used in high temperature conditions.